Sr Isotope Systematics of Aragonite Shell Fragments and Pore Waters from an ANDRILL Core, Southern McMurdo Sound, Antarctica

In December of 2007, the ANDRILL (ANtarctic Geological DRILLing program) Southern McMurdo Sound project recovered core AND-2A. The goal of the project was to reconstruct ice sheet and climate variation in Antarctica during the Neogene, and in doing so, obtain a clearer perspective on polar climate evolution. Initial attempts to apply Sr isotope stratigraphy to the recovered shell material in the core produced mixed results. Well-preserved calcitic shell fragments returned accurate ages and helped resolve some of the uncertainties of the age model, while seemingly unaltered aragonite shells invariably produced older than expected ages. To try to reconcile these results, additional analyses of the aragonite shells were carried out, and 87Sr/86Sr compositions of pore waters were measured. X-ray microdiffraction confirmed that only aragonite is present in the shells. In addition, electron microprobe element mapping revealed that Sr concentration in the aragonites defines clear growth bands, strongly suggesting original incorporation of Sr as a function of seasonal temperature variation. Although pore waters exhibit an otherwise highly modified chemistry, pore water 87Sr/86Sr compositions in aragonite shell-bearing intervals are in general good agreement with Sr isotope compositions of calcite. Calcite and pore water compositions also match 87Sr/86Sr ratios predicted by the core’s age model. By contrast, the observed departures of the aragonite from the predicted values are difficult to reconcile and cannot be thus far ascribed to equilibration with modified pore waters. Rather, the values observed in the aragonitic shell material must represent original, though presently unexplained anomalies. Other lines of evidence continue to be explored.